Origin of the early Permian Wajilitag igneous complex and associated Fe–Ti oxide mineralization in the Tarim large igneous province,NW China

The Wajilitag igneous complex is part of the early Permian Tarim large igneous province in NW China, and is composed of a layered mafic–ultramafic intrusion and associated syenitic plutons. In order to better constrain its origin, and the conditions of associated Fe–Ti oxide mineralization, we carried out an integrated study of mineralogical, geochemical and Sr–Nd–Hf isotopic analyses on selected samples. The Wajilitag igneous rocks have an OIB-like compositional affinity, similar to the coeval mafic dykes in the Bachu region. The layered intrusion consists of olivine clinopyroxenite, coarse-grained clinopyroxenite, fine-grained clinopyroxenite and gabbro from the base upwards. Fe–Ti oxide ores are mainly hosted in fine-grained clinopyroxenite. Forsterite contents in olivines from the olivine clinopyroxenite range from 71 to 76 mol%, indicating crystallization from an evolved magma. Reconstructed composition of the parental magma of the layered intrusion is Fe–Ti-rich, similar to that of the Bachu mafic dykes. Syenite and quartz syenite plutons have ε_Nd(t) values ranging from +1.4 to +2.9, identical to that for the layered intrusion. They may have formed by differentiation of underplated magmas at depth and subsequent fractional crystallization. Magnetites enclosed in olivines and clinopyroxenes have Cr₂O₃ contents higher than those interstitial to silicates in the layered intrusion. This suggests that the Cr-rich magnetite is an early crystallized phase, whereas interstitial magnetite may have accumulated from evolved Fe–Ti-rich melts that percolated through a crystal mush. Low V content in Cr-poor magnetite (<6600 ppm) is consistent with an estimate of oxygen fugacity of FMQ + 1.1 to FMQ + 3.5. We propose that accumulation of Fe–Ti oxides during the late stage of magmatic differentiation may have followed crystallization of Fe–Ti-melt under high fO₂ and a volatile-rich condition.     

The Munali Ni sulfide deposit,southern Zambia: A multi-stage,mafic-ultramafic,magmatic sulfide-magnetite-apatite-carbonate megabreccia

The Munali Intrusive Complex (MIC) is a flattened tube-shaped, mafic-ultramafic intrusion located close to the southern Congo Craton margin in the Zambezi belt of southern Zambia. It is made up of a Central Gabbro Unit (CGU) core, surrounded by a Marginal Ultramafic-mafic Breccia Unit (MUBU), which contains magmatic Ni sulfide mineralisation. The MIC was emplaced into a sequence of metamorphosed Neoproterozoic rift sediments and is entirely hosted within a unit of marble. Munali has many of the characteristics of craton-margin, conduit-style, dyke-sill complex-hosted magmatic sulfide deposits. Three-dimensional modelling of the MUBU on the southern side of the MIC, where the Munali Nickel Mine is located, reveals a laterally discontinuous body located at the boundary between footwall CGU and hangingwall metasediments. Mapping of underground faces demonstrates the MUBU to have intruded after the CGU and be a highly complex, multi stage megabreccia made up of atypical ultramafic rocks (olivinites, olivine-magnetite rocks, and phoscorites), poikilitic gabbro and olivine basalt/dolerite dykes, brecciated on a millimetre to metre scale by magmatic sulfide. The breccia matrix is largely made up of a sulfide assemblage of pyrrhotite-pentlandite-chalcopyrite-pyrite with varying amounts of magnetite, apatite and carbonate. The sulfides become more massive towards the footwall contact. Late stage, high temperature sulfide-carbonate-magnetite veins cut the rest of the MUBU. The strong carbonate signature is likely due, in part, to contamination from the surrounding marbles, but may also be linked to a carbonatite melt related to the phoscorites. Ductile deformation and shear fabrics are displayed by talc-carbonate altered ultramafic clasts that may represent gas streaming textures by CO₂-rich fluids. High precision U-Pb geochronology on zircons give ages of 862.39 ± 0.84 Ma for the poikilitic gabbro and 857.9 ± 1.9 Ma for the ultramafics, highlighting the multi-stage emplacement but placing both mafic and later ultramafic magma emplacement within the Neoproterozoic rifting of the Zambezi Ocean, most likely as sills or sheet-like bodies. Sulfide mineralisation is associated with brecciation of the ultramafics and so is constrained to a maximum age of 858 Ma. The Ni- and Fe-rich nature of the sulfides reflect either early stage sulfide saturation by contamination, or the presence of a fractionated sulfide body with Cu-rich sulfide elsewhere in the system. Munali is an example of a complex conduit-style Ni sulfide deposit affected by multiple stages and sources of magmatism during rifting at a craton margin, subsequent deformation; and where mafic and carbonatitic melts have interacted along deep seated crustal fault systems to produce a mineralogically unusual deposit.     

Mineral chemistry of high-Mg diorites and skarn in the Han-Xing Iron deposits of South Taihang Mountains,China: Constraints on mineralization process

The Han-Xing region is located in the south Taihang Mountains (TM) in the central part of the North China Craton, and is an important iron producing area. The iron deposits in this region are of skarn type, related to an Early Cretaceous high-Mg diorite complex, including gabbro diorite, hornblende diorite, diorite, diorite porphyrite, and monzonite. In this study we report the detailed mineral chemistry of the high-Mg diorites and skarn rocks. The olivine in the gabbro diorite shows chemical composition similar to that in mantle peridotite xenoliths. Clinopyroxene in the gabbro diorite is dominantly augite, with only minor diopside, whereas the clinopyroxenes in the diorite and monzonite are diopside. Amphiboles in the high-Mg diorites show compositional range from magnesiohornblende to magnesiohastingsite, with minor pargasite and tschermakite. Most plagioclase in the high-Mg diorite is andesine and oligoclase. The magnesio-biotite in gabbro diorites shows chemical characteristics of re-equilibrated primary biotites and those in calc-alkaline rocks. In the diorite and diorite porphyrite, plagioclase shows complex chemical zoning. Clinopyroxene and garnet in skarn rocks show varying FeO contents, the former containing low FeO (< 9 wt.%) and occurring as the major skarn mineral in large-scale iron deposits, and the latter within small-scale iron deposits with high FeO (mostly > 25 wt.%) content. We computed the pressure, temperature, oxygen fugacity and water contents based on the mineral chemistry of amphibole and biotite. Based on the results, the magma crystallization can be divided into two stages, one within the deep magma chamber, forming clinopyroxene, amphibole and plagioclase phenocrysts; the other after emplacement, forming the rim of phenocrysts and matrix minerals. The magma during the early stage shows high temperature (~ 900 °C–950 °C), pressure (~ 300 MPa–500 MPa), relatively high logfO₂ (NNO–NNO + 2), and H₂O content in melt (4%–8%). During the late stage, the magma temperature dropped to about 750 °C, and pressure came down to less than 100 MPa, with the logfO₂ rising to NNO + 1–NNO + 2.The zoning of amphibole and plagioclase records the process of magma mixing and crystallization, with injection of mafic magma into the felsic magma chamber. The relatively high logfO₂ and H₂O content inhibited partitioning of iron into mafic minerals and favored concentration of Fe in the melt. Iron ore precipitation occurred when the magma was emplaced at shallow level, and was principally controlled by the chemical composition of carbonate wall rocks. The high logfO₂, Fe^3 + rich ore-forming fluid generated andradite and clinopyroxene when it reacted with limestone and dolomitic limestone respectively.     

新疆磁海铁矿区镁铁质岩及正长岩锆石U-Pb年代学、岩石地球化学特征:对成岩、成矿作用的制约

对新疆磁海铁矿区镁铁-超镁铁质岩与铁成矿关系、正长岩与镁铁质岩关系的解剖,是认识磁海矿区成岩、成矿过程及构造背景的关键。本文利用SIMS锆石U-Pb测年法,获得磁海辉绿岩、辉长辉绿岩、磁南辉长岩、磁海北角闪石英正长岩的206Pb/238U-207Pb/235U谐和年龄分别为275.1±2.2Ma、281.9±3.2Ma、273.0±1.9Ma和273.0±1.8Ma,这与北山乃至北疆地区主要含铜镍-钒钛磁铁矿的镁铁-超镁铁质岩年龄一致。岩石地球化学特征研究显示,从辉石岩到辉长岩,再到辉绿岩,经历了Ti逐渐富集、Mg#和m/f值先增加后降低的过程,角闪石英正长岩具有A型花岗岩特征,与辉长岩、辉绿岩在成因上存在互补关系。综合年代学和地球化学特征,磁南辉石岩、辉长岩、磁海辉绿岩、辉长辉绿岩以及磁海北边的角闪石英正长岩为同源岩浆演化的产物,岩浆演化过程中受地壳混染作用微弱,在岩浆演化的早期,磁铁矿的结晶分离主导着岩浆成分的改变,当岩浆演化到辉长岩阶段,岩浆开始以结晶分异作用为主;磁铁矿的分离结晶时间早于钛铁矿,岩浆型的金属硫化物为磁铁矿和钛铁矿结晶过渡阶段的产物。磁海镁铁-超镁铁质岩石在成岩及成矿作用上可能与在时间和空间上相邻近的塔里木早二叠世大火成岩省有密切关系。     

Postdated melting of subcontinental lithospheric mantle by the Emeishan mantle plume: Evidence from the Anyi intrusion,Yunnan, SW China

The Anyi intrusion is located in the central zone of Emeishan large igneous province (ELIP), SW China. It outcrops in an area of about 0.65 km² and ~ 1 km thick and dips to the southwest. The Anyi intrusion consists of a lower clinopyroxenite zone, middle gabbro zone, and an upper monzonite–syenite zone. Up to 400 m thick stratiform disseminated Fe–Ti oxide layer with grades of 16–18 wt.% total Fe is hosted in the lower clinopyroxenite zone. Zircon SHRIMP U–Pb age (247 ± 3 Ma) indicates that the Anyi intrusion represents postdated mafic magmatism resulting from the ~ 260 Ma Emeishan mantle plume. Compared with the typical oxide-bearing intrusions (such as Panzhihua and Baima) formed at ~ 260 Ma in the ELIP, the Anyi intrusion is characterized by high alkaline contents and LREE/HREE ratios, extremely low εNd values (− 6.2 to − 7.6) and moderate high (⁸⁷Sr/⁸⁶Sr)_i values (0.7072 to 0.7086). These characteristics of the Anyi intrusion cannot be explained by fractional crystallization or crustal contamination, but may reflect a unique enriched continental lithospheric mantle source (a mantle source mixed between garnet pyroxenite and spinel peridotite). We propose that the postdated mafic magmatism associated with the formation of the Anyi intrusion and its Fe–Ti oxide ore may be the product of melting of a mantle source mixed between garnet pyroxenite and spinel peridotite in the shallow lithosphere caused by conductive heating combined with lithosphere thinning due to plume–lithosphere interaction.     

Fractional crystallization and the formation of thick Fe–Ti–V oxide layers in the Baima layered intrusion,SW China

The Baima layered intrusion is located in the central part of the Emeishan Large Igneous Province (ELIP). The N–S striking intrusion is ~ 24 km long and ~ 2 km thick and dips to the west. Based on variations in modal proportions and cumulus mineral assemblages, the intrusion from the base to the top is simply subdivided into a lower zone (LZ) with most of the economic magnetite layers, and an upper zone (UZ) with apatite-bearing troctolite and gabbro. The rock textures suggest crystallization of the Fe–Ti oxide slightly later than plagioclase (An_67-54) but relatively earlier than olivine (Fo_74-55), followed by clinopyroxene and finally apatite.Relatively low olivine forsterite content and abundant ilmenite exsolution lamellae in clinopyroxene indicate that the Baima parental magma is a highly evolved Fe–Ti-rich magma. Via MELTS model, it demonstrates that under a closed oxygen system, extensive silicate mineral fractionation of a picritic magma might lead to Fe and Ti enrichment and oxygen fugacity elevation in the residual magma. When such Fe–Ti-rich magma ascends to the shallower Baima intrusion, the Fe–Ti oxides may become an early liquidus phase. Well-matched olivine and plagioclase microprobe data with the results of MELTS calculation, combined with relatively low CaO content in olivine (0.02–0.08 wt.%) indicate that wall-rock contamination probably plays a weak role on oxygen fugacity elevation and the early crystallization of Fe–Ti oxides. Several reversals in whole-rock chromium and plagioclase anorthite contents illustrate that multiple recharges of such Fe–Ti-rich magma mainly occurred along the lower part of the Baima magma chamber. Frequent Fe–Ti-rich magma replenishment and gravitational sorting and settling are crucial for the development of thick Fe–Ti oxide layers at the base of the Baima layered intrusion.     

New textural and mineralogical constraints on the origin of the Hongge Fe-Ti-V oxide deposit, SW China

The Hongge magmatic Fe-Ti-V oxide deposit in the Panxi region, SW China, is hosted in a layered mafic–ultramafic intrusion. This 2.7-km-thick, lopolith-like intrusion consists of the lower, middle, and upper zones, which are composed of olivine clinopyroxenite, clinopyroxenite, and gabbro, respectively. Abundant Fe-Ti oxide layers mainly occur in the middle zone and the lower part of the upper zone. Fe-Ti oxides include Cr-rich and Cr-poor titanomagnetite and granular ilmenite. Cr-rich titanomagnetite is commonly disseminated in the olivine clinopyroxenite of the lower parts of the lower and middle zones and contains 1.89 to 14.9 wt% Cr₂O₃ and 3.20 to 16.2 wt% TiO₂, whereas Cr-poor titanomagnetite typically occurs as net-textured and massive ores in the upper middle and upper zones and contains much lower Cr₂O₃ (<0.4 wt%) but more variable TiO₂ (0.11 to 18.2 wt%). Disseminated Cr-rich titanomagnetite in the ultramafic rocks is commonly enclosed in either olivine or clinopyroxene, whereas Cr-poor titanomangetite of the net-textured and massive ores is mainly interstitial to clinopyroxene and plagioclase. The lithology of the Hongge intrusion is consistent with multiple injections of magmas, the lower zone being derived from a single pulse of less differentiated ferrobasaltic magma and the middle and upper zones from multiple pulses of more differentiated magmas. Cr-rich titanomagnetite in the disseminated ores of the lower and middle zones is interpreted to represent an early crystallization phase whereas clusters of Cr-poor titanomagnetite, granular ilmenite, and apatite in the net-textured ores of the middle and upper zones are thought to have formed from an Fe-Ti-(P)-rich melt segregated from a differentiated ferrobasaltic magma as a result of liquid immiscibility. The dense Fe-Ti-(P)-rich melt percolated downward through the underlying silicate crystal mush to form net-textured and massive Fe-Ti oxide ores, whereas the coexisting Si-rich melt formed the overlying plagioclase-rich rocks in the intrusion.     

Apatite geochemistry of the Taihe layered intrusion,SW China: Implications for the magmatic differentiation and the origin of apatite-rich Fe-Ti oxide ores

The Taihe intrusion is one of the layered intrusions situated in the central zone of the Emeishan Large Igneous Province (ELIP), SW China. The cyclic units in the Middle Zone of the intrusion are composed of apatite-magnetite clinopyroxenite at the base and gabbro at the top. The apatite-rich oxide ores contain 6–12 modal% apatite and 20–50 modal% Fe-Ti oxides evidently distinguished from the coeval intrusions in which apatite-rich rocks are poor in Fe-Ti oxides. Most of apatites of the Taihe Middle and Upper Zones are fluorapatite, although four samples show slightly high Cl content in apatite suggesting that they crystallize from a hydrous parental magma. Compared to the apatite from the gabbro of the Panzhihua intrusion, situated 100 km to the south of the Taihe intrusion, the apatite of the Taihe rocks is richer in Sr and depleted in HREE relative to LREE. The calculated magma in equilibrium with apatite of the Taihe Middle and Upper Zones also shows weakly negative Sr anomalies in primitive mantle normalized trace element diagrams. These features indicate that the apatite of the Taihe Middle and Upper Zones crystallizes after clinopyroxene and before plagioclase. The apatite of the Taihe Middle and Upper Zones shows weakly negative Eu anomalies suggesting a high oxygen fugacity condition. The high iron and titanium contents in the oxidizing magma result in crystallization of Fe-Ti oxides. Crystallization of abundant Fe-Ti oxides and clinopyroxenes lowers the solubility of phosphorus and elevates SiO₂ concentration in the magma triggering the saturation of apatite. The positive correlations of Sr, V, total REE contents and Ce/Yb ratio in apatite with cumulus clinopyroxene demonstrate approximately compositional equilibrium between these phases suggesting they crystallized from the same ferrobasaltic magma. Early crystallization and accumulation of Fe-Ti oxide together with apatite produced the apatite-rich oxide ores at the base of the cyclic units of the Taihe Middle Zone.     

The Cihai diabase in the Beishan region,NW China: Isotope geochronology,geochemistry and implications for Cornwall-style iron mineralization

Diabase dykes in Cihai, Beishan region, NW China are spatially and temporally associated with ‘Cornwall-type’ iron deposits. U–Pb dating of zircons from a diabase dyke using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields an age of 128.5 ± 0.3 Ma, indicating an Early Cretaceous crystallization age. Most of the diabases show low Mg-numbers, suggesting evolved magmas. The diabase dykes show typical ophitic or sub-ophitic textures, and are dominantly composed of phenocrysts of plagioclase (40–50%) and clinopyroxene (30–45%), with minor and varying amounts of biotite and hornblende (1–5%), and minor disseminated magnetite (∼5%). Their mineralogy reflects magma differentiation under relatively low oxygen fugacity conditions. The diabase dykes are characterized by minor variation in SiO₂ (44.67–49.76 wt.%) and MnO (0.14–0.26 wt.%), but show a marked range of Al₂O₃ (10.66–14.21 wt.%), total Fe₂O₃ (9.52–13.88 wt.%), TiO₂ (0.66–2.82 wt.%) and relatively high MgO (4.87–9.29 wt.%) with an Mg# value [atomic Mg/(Mg + Fe²⁺)] of up to 66. The Cihai diabases possibly experienced fractional crystallization of olivine + clinopyroxene and minor crustal contamination during the differentiation process. Prominent negative Nb, Ta and Ti anomalies suggest derivation from subduction-modified mantle. Furthermore, the rocks have relatively unradiogenic Sr- and Nd-isotopic ratios. These characteristics probably reflect partial melting of a subduction component in the source mantle lithosphere through heat input from an upwelling asthenospheric mantle. Such processes probably occurred within an extensional setting during the Early Cretaceous in the Beishan area. The iron-rich fluids were derived from deep sources, and the iron ores were concentrated through a convection cell driven by temperature gradients established by the intrusion of the diabase sills. The combined processes of subduction-related enrichment in the source, shallow depth of emplacement, and the involvement of large-scale circulation of basinal brines from an evaporitic source are inferred to have contributed to the formation of the ‘Cornwall-type’ mineralization in Cihai.     

Petrogenesis of Dongguashan skarn-porphyry Cu-Au deposit related intrusion in the Tongling district,eastern China: Geochronological,mineralogical, geochemical and Hf isotopic evidence

The Dongguashan skarn-porphyry Cu-Au deposit, located in the Tongling district of the Middle-Lower Yangtze River Valley metallogenic belt (MLYB), consists of skarn ore bodies in the upper part and porphyry ore bodies in the lower part, both of which are hosted in quartz diorite and quartz monzodiorite. Zircon U-Pb age and geochemical studies show that the quartz diorite of the Dongguashan intrusion formed at 140.3 ± 2.0 Ma (MSWD = 0.19) and belongs to the high potassium calc-alkaline series. It is enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), depleted in high field-strength elements (HFSE) and heavy rare earth elements (HREE), and has a slightly negative Eu anomaly. ¹⁷⁶Hf/¹⁷⁷Hf values of the rims of zircons show a variable range (0.282087–0.282391), corresponding with calculated ε_Hf(t) values of − 10.72 to − 21.46. Plagioclases in the quartz diorite have unbalanced structure characterized by bright andesine and labradorite (An = 37.0–65.5) cores with higher contents of Fe and Sr and are corroded by dark oligoclase (An = 13.8–27.6) rim. Major elements, trace elements, Hf isotope, and the composition of plagioclases indicate that the parental magma of the Dongguashan intrusion was produced by the mixing of underplating mafic magma and felsic magma formed by remelting of Paleoproterozoic and Neoarchean crustal rocks, Neoproterozoic crust may also provide some material to the felsic magma. Mafic magma played a key role and made the parental magma rich in water, sulfur, metals (Cu, Au) and gave it a high oxygen fugacity. During its magmatic evolution, the parental magma underwent fractional crystallization of hornblende, apatite, sphene and other mafic minerals. Some quartz diorite and quartz monzodiorite samples that show adakitic signatures, may result from injection of mafic magma. Some inherited zircons of the quartz diorite in the Dongguashan intrusion gave ages of 2.40–2.50 Ga, 1.95–2.05 Ga and 0.74–0.81 Ga, coming from ultramafic, mafic and andesitic igneous rocks, and this indicates that there may have been three periods (2.4, 2.0, and 0.8 Ga) of magmatic activity in the Tongling district.     

Geochemical modelling of the Chilas Complex in the Kohistan Terrane,northern Pakistan

The Chilas Complex in the Kohistan Terrane, Pakistan, is a huge basic intrusion, about 300 km long and up to 40 km wide, which is regarded as tilted island-arc type crust. It has been interpreted as the magma chamber root zone of the Kohistan Island Arc. The Chilas Complex is composed mainly of gabbronorite (main facies) and several masses of ultramafic–mafic–anorthosite (UMA) association. The UMA association consists mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite), with minor amount of pyroxene-dominant cumulate (clinopyroxenite, websterite).The major element geochemistry of the gabbronorite (main facies) and rocks of the UMA association, plotted on Harker diagrams, are explained by a cumulate and a non-cumulate model, respectively. Namely, the UMA association is explained as variable crystal cumulates from a primary magma and the gabbronorite of the main facies is explained as due to the fractionation of the residual melt. Chemical variations of major, trace and rare earth elements for the gabbronorite of the main facies in the Chilas Complex are explained by fractional crystallization and accumulation of plagioclase, orthopyroxene and clinopyroxene from the residual melt of the primary magma.     

Petrology,geochemistry and geochronology of the magmatic suite from the Jianzha Complex,central China: Petrogenesis and geodynamic implications

The intermediate–mafic–ultramafic rocks in the Jianzha Complex (JZC) at the northern margin of the West Qinling Orogenic Belt have been interpreted to be a part of an ophiolite suite. In this study, we present new geochronological, petrological, geochemical and Sr–Nd–Hf isotopic data and provide a different interpretation. The JZC is composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro, and pyroxene diorite. The suite shows characteristics of Alaskan-type complexes, including (1) the low CaO concentrations in olivine; (2) evidence of crystal accumulation; (3) high calcic composition of clinopyroxene; and (4) negative correlation between FeO^tot and Cr₂O₃ of spinels. Hornblende and phlogopite are ubiquitous in the wehrlites, but minor orthopyroxene is also present. Hornblende and biotite are abundant late crystallized phases in the gabbros and diorites. The two pyroxene-bearing diorite samples from JZC yield zircon U–Pb ages of 245.7 ± 1.3 Ma and 241.8 ± 1.3 Ma. The mafic and ultramafic rocks display slightly enriched LREE patterns. The wehrlites display moderate to weak negative Eu anomalies (0.74–0.94), whereas the olivine gabbros and gabbros have pronounced positive Eu anomalies. Diorites show slight LREE enrichment, with (La/Yb)_N ratios ranging from 4.42 to 7.79, and moderate to weak negative Eu anomalies (Eu/Eu¹ = 0.64–0.86). The mafic and ultramafic rocks from this suite are characterized by negative Nb–Ta–Zr anomalies as well as positive Pb anomalies. Diorites show pronounced negative Ba, Nb–Ta and Ti spikes, and typical Th–U, K and Pb peaks. Combined with petrographic observations and chemical variations, we suggest that the magmatism was dominantly controlled by fractional crystallization and crystal accumulation, with limited crustal contamination. The arc-affinity signature and weekly negative to moderately positive ε_Nd(t) values (−2.3 to 1.2) suggest that these rocks may have been generated by partial melting of the juvenile sub-continental lithospheric mantle that was metasomatized previously by slab-derived fluids. The lithologies in the JZC are related in space and time and originated from a common parental magma. Geochemical modeling suggests that their primitive parental magma had a basaltic composition. The ultramafic rocks were generated through olivine accumulation, and variable degrees of fractional crystallization with minor crustal contamination produced the diorites. The data presented here suggest that the subduction in West Qinling did not cease before the early stage of the Middle Triassic (∼242 Ma), a back-arc developed in the northern part of West Qinling during this period, and the JZC formed within the incipient back-arc.     

Mineralogy,fluid inclusions,and isotopes of the Cihai iron deposit,eastern Tianshan,NW China: Implication for hydrothermal evolution and genesis of subvolcanic rocks-hosted skarn-type deposits

Most skarn deposits are closely related to granitoids that intruded into carbonate rocks. The Cihai (>100 Mt at 45% Fe) is a deposit with mineral assemblages and hydrothermal features similar to many other typical skarn deposits of the world. However, the iron orebodies of Cihai are mainly hosted within the diabase and not in contact with carbonate rocks. In addition, some magnetite grains exhibit unusual relatively high TiO₂ content. These features are not consistent with the typical skarn iron deposit. Different hydrothermal and/or magmatic processes are being actively investigated for its origin. Because of a lack of systematic studies of geology, mineral compositions, fluid inclusions, and isotopes, the genetic type, ore genesis, and hydrothermal evolution of this deposit are still poorly understood and remain controversial.The skarn mineral assemblages are the alteration products of diabase. Three main paragenetic stages of skarn formation and ore deposition have been recognized based on petrographic observations, which show a prograde skarn stage (garnet-clinopyroxene-disseminated magnetite), a retrograde skarn stage (main iron ore stage, massive magnetite-amphibole-epidote ± ilvaite), and a quartz-sulfide stage (quartz-calcite-pyrite-pyrrhotite-cobaltite).Overall, the compositions of garnet, clinpyroxene, and amphibole are consistent with those of typical skarn Fe deposits worldwide. In the disseminated ores, some magnetite grains exhibit relatively high TiO₂ content (>1 wt.%), which may be inherited from the diabase protoliths. Some distinct chemical zoning in magnetite grains were observed in this study, wherein cores are enriched in Ti, and magnetite rims show a pronounced depletion in Ti. The textural and compositional data of magnetite confirm that the Cihai Fe deposit is of hydrothermal origin, rather than associated with iron rich melts as previously suggested.Fluid inclusions study reveal that, the prograde skarn (garnet and pyroxene) formed from high temperature (520–600 °C), moderate- to high-salinity (8.1–23.1 wt.% NaCl equiv, and >46 wt.% NaCl equiv) fluids. Massive iron ore and retrograde skarn assemblages (amphibole-epidote ± ilvaite) formed under hydrostatic condition after the fracturing of early skarn. Fluids in this stage had lower temperature (220°–456 °C) and salinity (8.4–16.3 wt.% NaCl equiv). Fluid inclusions in quartz-sulfide stage quartz and calcite also record similar conditions, with temperature range from 128° to 367 °C and salinity range from 0.2 to 22.9 wt.% NaCl equiv. Oxygen and hydrogen isotopic data of garnet and quartz suggest that mixing and dilution of early magmatic fluids with external fluids (e.g., meteoric waters) caused a decrease in fluid temperature and salinity in the later stages of the skarn formation and massive iron precipitation. The δ¹⁸O values of magnetite from iron ores vary between 4.1 and 8.5‰, which are similar to values reported in other skarn Fe deposits. Such values are distinct from those of other iron ore deposits such as Kiruna-type and magmatic Fe-Ti-V deposits worldwide. Taken together, these geologic, geochemical, and isotopic data confirm that Cihai is a diabase-hosted skarn deposit related to the granitoids at depth.     

Keketuohai mafic–ultramafic complex in the Chinese Altai,NW China: Petrogenesis and geodynamic significance

Devonian magmatism was very intensive in the tectonic evolutionary history of the Chinese Altai, a key part of the Central Asian Orogenic Belt (CAOB). The Devonian Keketuohai mafic–ultramafic complex in the Chinese Altai is a zoned intrusion consisting of dunite, olivine gabbro, hornblende gabbro and pyroxene diorite. The pyroxene diorite gives a zircon U–Pb age of 409 ± 5 Ma. Variations in mineral assemblage and chemical composition suggest that the petrogenesis of the Keketuohai Complex was chiefly governed by fractional crystallization from a common magma chamber. Low SiO₂, K₂O and Na₂O contents, negative covariations between P₂O₅, TiO₂ and Mg# value suggest insignificant crustal assimilation/contamination. Thus the positive ε_Nd(t) values (0 to + 2.7) and slight enrichments in light rare earth elements (e.g., La/Yb_N = 0.98–3.64) suggest that their parental magma was possibly produced by partial melting of the lithospheric mantle. Model calculation suggests that their parental magma was high-Mg (Mg# = 66) tholeiitic basaltic melt. The Keketuohai intrusion was coeval with diverse magmatism, high temperature metamorphism and hydrothermal mineralization, which support a previously proposed model that ridge subduction most likely played an important role in the tectonic evolution of the Chinese Altai.     

Precious metal mineralisation in the Sotkavaara Intrusion,northern Finland: Peak Pt,Pd, Au and Cu offsets in a small intrusion with poorly-developed magmatic layering

The Sotkavaara Intrusion is a small (2.5 × 1.5 km surface expression) mafic-ultramafic intrusion located in northern Finland, 25 km east-southeast of Rovaniemi. The intrusion was drilled by the Geological Survey of Finland between 2009 and 2012, when low-S, low-grade precious metal mineralisation (<1.1 ppm Pt + Pd + Au) was discovered. Emplaced into the 1.98 Ga eastern Peräpohja Schist Belt, Sotkavaara occurs a few tens of kilometres north of the 2.44 Ga the Penikat, Portimo and Koillismaa intrusions, which together host Europe’s most significant platinum-group element (PGE) mineralisation. The intrusion comprises a volumetrically subordinate Gabbro Unit and volumetrically dominant Pyroxenite Unit, within which layering is poorly-developed. The Pyroxenite Unit is composed predominantly of clinopyroxenite and contains a small number of laterally discontinuous wehrlite, olivine clinopyroxenite and low-Cr clinopyroxenite (<0.1 wt% Cr₂O₃) layers. Precious metal mineralisation is hosted by low-Cr clinopyroxenite layers and shows systematic Pt, Pd, Au and Cu offsets similar to those in the Munni Munni, Rincón del Tigre, Skaergaard and Great Dyke intrusions. Relative to these global examples, the Sotkavaara Intrusion represents an atypical occurrence, exhibiting poorly-developed modal layering in a small mafic-ultramafic intrusion, alongside moderately heavy δ ³⁴S values (+1.3 to +9.6‰). Despite this, mineralisation can be broadly attributed to similar ore-forming processes, whereby in-situ fractional crystallisation is ultimately responsible for generating small volumes of sulphide melt. Although mineralisation is sub-economic, this occurrence indicates that northern Finland may be prospective for magmatic sulphide deposits in Svecofennian-age mafic-ultramafic intrusions. Globally, Sotkavaara highlights that well-developed modal layering and large magma chambers are not necessarily requirements for forming this type of mineralisation.     

Cenozoic forearc gabbros from the northern zone of the Eastern Pontides Orogenic Belt,NE Turkey: Implications for slab window magmatism and convergent margin tectonics

The Eastern Pontides Orogenic Belt represents one of the best examples of fossil convergent margins in the eastern Mediterranean region. However, the origin and geodynamic setting of the late Mesozoic–Cenozoic magmatism in this belt remain controversial due to lack of systematic geological, geochemical and chronological data. The general consensus is that the late Mesozoic–Cenozoic igneous activity is related to northward subduction of oceanic lithosphere in the late Mesozoic and following collision between Tauride and Pontide blocks in the early Cenozoic. Here we present a comprehensive study focusing on the origin and geodynamic setting of gabbro bodies exposed along a narrow zone, parallel to the southeastern coast of the eastern Black Sea basin, in the Northern Zone of the Eastern Pontides Orogenic Belt.The studied gabbro bodies are hosted within late Cretaceous basaltic, andesitic, and dacitic volcanics including pyroclastic rocks and interbedded sedimentary rocks. The gabbro bodies range in size from 0.1 km² to 1.5 km², and outcrop patterns vary from round or elliptical to markedly elongate with sharp and discordant contact with the host rocks. Their mineral assemblage includes mainly clinopyroxene, plagioclase, minor olivine, amphibole, magnetite and rarely orthopyroxene, biotite, zircon and titanite. The occurrence of sutured grain boundaries on clinopyroxene and plagioclase, and the presence of reverse compositional zoning in clinopyroxene and olivine suggest mixing between magmas of contrasting compositions during mineral growth. Thermobarometric computations indicate that the temperature at the beginning of crystallization was ~ 1250 °C and crystallization was polybaric. Zircon and titanite U–Pb ages indicate that these small intrusions were emplaced into crustal rocks of the Eastern Pontides Orogenic Belt during Lutetian (45 ± 2 Ma). The depletion of HFSE is consistent with the involvement of an arc-related source in the petrogenesis of these rocks, and low to moderate enrichment Ce, Rb, Ba, K, Pb, Sr and Th suggests that involvement of subducted oceanic sediment was modest. The low Th content and low Th/Yb indicate that the role of sediment addition was nevertheless limited. The Nd, Sr and Pb isotopic data are consistent with the interpretation that the dominant source component in these gabbros is a depleted, peridotitic mantle, and that crustal contamination is relatively unimportant. We suggest that mafic magmas that produced the gabbroic intrusions were derived from melting of a depleted mantle source under the forearc region of the Eastern Pontides Orogenic Belt during southward subduction of two oceanic plates separated by a mid-ocean ridge, leading to the formation of a slab window. We also infer fractional crystallization and assimilation during both magma storage in the crust–mantle transition zone and transfer into the overlying arc crust.     

The post-collisional Cihai iron skarn deposit,eastern Tianshan,Xinjiang, China

The Cihai iron skarn deposit is located in the southern part of the eastern Tianshan, Xinjiang, northwestern China. The major iron orebodies are banded and nearly parallel to each other. The iron ores are hosted in an early diabase dike and in skarn. Post-ore diabase dikes cut the iron ores and their hosting diabase. Hydrothermal activity can be divided into four stages based on geological and petrographic observations: initial K–Na alteration (stage I), skarn-minor magnetite event (II), retrograde skarn-magnetite main ore event (III), and quartz–calcite–sulfide veining (IV). Zircon U–Pb dating yields ages of 286.5 ± 1.8 Ma for early diabase and 275.8 ± 2.2 Ma for post-ore diabase dikes. Amphibole separated from massive magnetite ore gives a ⁴⁰Ar–³⁹Ar plateau age of 281.9 ± 2.2 Ma and is the time of ore formation. Formation of the Cihai iron deposit is closely related to post-collisional magmatism and associated Cu–Ni–Au polymetallic mineralization in the eastern Tianshan.     

The geology,geochemistry and Ni-Cu-PGE potential of mafic-ultramafic bodies associated with the Dido Batholith,North Queensland,Australia

Four elongate, km-scale, mafic to ultramafic bodies (UMB) were identified within the Ordovician-Silurian Dido Batholith, north Queensland, and were assessed for Ni-Cu-PGE fertility. The UMB comprise layered cumulate sequences and represent open-system intrusions emplaced at mid-crustal levels. The UMB are divided into two petrographically and geochemically distinct types: (1) low-Fe UMB (3 intrusions), comprising dunites, wehrlites, troctolites and olivine gabbro which contain variable amounts of olivine (Fo_85–72), clinopyroxene (Mg# 0.87–0.73), plagioclase (An_92–72) and chromites; and (2) high-Fe UMB (1 intrusion), comprising dunites, wehrlites and pyroxenites which lack chromites, contain abundant early crystallising Fe-Ti oxides and hornblende, and less primitive olivines (Fo_78–72) and pyroxenes (Mg# 0.87–0.73). The calculated parent magmas of the low-Fe UMB contained 8–10 wt% of both MgO and FeO_t, whereas the high-Fe UMB parent magmas were more evolved, having higher FeO_t (12–16 wt%) and lower MgO (6.2–8.2 wt%), Ni and Cr concentrations. The parent magmas of the Dido UMB are interpreted as mantle-derived arc rift or backarc tholeiites, and mineral compositions suggest the UMB are similar to arc-cumulate, Klamath-type intrusions. Crustal contamination during ascent is suggested to be responsible for the LREE-enriched and Nb- and Ti-depleted nature of the UMB parent magmas. A two-component Sr-Nd isotope mixing model suggests that the addition of variable amounts (<5% in the low-Fe UMB and 9–10% in the high-Fe UMB) of 2000–2500 Ma igneous crustal contaminant to tholeiitic melts derived from a slightly enriched mantle source can account for isotopic compositions of the UMB. Although the mantle-derived, crustally contaminated nature of the parent magmas are positive factors for magmatic sulphide mineralisation, economic mineralisation has not been found associated with the UMB. Geochemical discriminators, such as Cu/Pd and Ni-depletion, suggest that the magmas that formed the km-scale UMB were chalcophile element-depleted, having undergone a previous S-saturation event (i.e., significant sulphide-deposition) at depth. The slight PGE-enrichment in the high-Fe cumulates (up to 160 ppb of both Pt and Pd) is suggested to be due to the addition of small amounts of PGE to the high-Fe magmas from an external source. There is no evidence to suggest that large volumes of PGE-enriched sulphides were added to the UMB magmas. These findings, in addition to the knowledge that most large deposits are associated with intracontinental settings rather than convergent settings, determined that the investigated Dido UMB are low-priority Ni-Cu-PGE targets. However, the likelihood that the Dido UMB were emplaced in local extensional regimes within subduction-related environments, i.e., a similar geodynamic setting to economic deposits such as Aguablanca (Spain), requires that other UMB in the Dido area are assessed on an individual basis.     

Platinum-group elemental chemistry of the Baima and Taihe Fe–Ti oxide bearing gabbroic intrusions of the Emeishan large igneous province,SW China

Nickel-, copper-, and platinum group element (PGE)-enriched sulphide mineralization in large igneous provinces has attracted numerous PGE studies. However, the distribution and behavior of PGEs as well as the history of sulphide saturation are less clear in oxide-dominated mineralization. Platinum group elements of oxide-bearing layered mafic intrusions from the Emeishan large igneous province are examined in this study. Samples collected from the Baima and Taihe oxide-bearing layered gabbroic intrusions reveal contrasting results. The samples from Baima gabbroic rocks have low total PGE abundances (ΣPGE < 4 ppb) whereas the Taihe gabbroic rocks, on average, have more than double the concentration but are variable ranging from ΣPGE < 2 ppb to ΣPGE ∼300 ppb. The Baima gabbro is platinum-subgroup PGE (PPGE = Rh, Pt and Pd) enriched and iridium-subgroup PGE (IPGE = Os, Ir and Ru) depleted, with a distinct positive Ru anomaly on a primitive mantle normalized multi-element plot. The Taihe gabbros are also PPGE enriched but with negative Ru and Pd anomalies on a primitive mantle normalized multi-element plot. The PGE concentrations of Baima rocks are indicative of fractionation of a relatively evolved, mafic, S-undersaturated parental magma that was affected by earlier sulphide segregation. In contrast, the Taihe rocks record evidence of both S-saturated and S-undersaturated conditions and that the parental magma was likely emplaced very close to S-saturation. Comparisons of the platinum group element contents in the Emeishan flood basalts and the Emeishan oxide-bearing intrusions suggest that the PGE budget in a magma is not controlled by magma series (high-Ti vs. low-Ti), but very much by crustal contamination. The unlikelihood of substantial crustal contamination in the Taihe magma allowed the magma to remain S-undersaturated for a longer duration. PGE and sulphide mineralization was not identified in the Taihe intrusion but the presence of one PGE-enriched sample (Pt + Pd = ∼300 ppb) suggests that the parental magma likely did not experience sulphide segregation and is a potential target for further prospecting.     

Early Carboniferous sub- to mid-alkaline magmatism in the Eastern Sierras Pampeanas,NW Argentina: A record of crustal growth by the incorporation of mantle-derived material in an extensional setting

A recently discovered granitic intrusion at Cerro La Gloria in western Sierra de Famatina (NW Argentina) is representative of sub- to mid-alkaline Carboniferous magmatism in the region. The main rock type consists of microcline, quartz and plagioclase, with amphibole, magnetite, ilmenite, biotite, epidote, zircon, allanite and sphene as accessory minerals. We report a U–Pb zircon SHRIMP age for the pluton of 349 ± 3 Ma (MSWD = 1.1), i.e., Tournaisian. Whole-rock chemical composition and Nd isotope analyses are compatible with an origin by melting of older mafic material in the lower crust (εNdt between − 0.58 and + 0.46 and T_DM values of about 1.1 Ga). The pluton is intruded by penecontemporaneous to late alkaline mafic dykes that are classified as back-arc basalts. Coeval, Early Carboniferous A-type granites occur farther east in the Sierras Pampeanas, probably generated during lithospheric stretching. Overall, the Early Carboniferous granitic rocks show a west-to-east mineralogical and isotopic zonation indicating that magma genesis involved a greater contribution of juvenile material of mantle character to the west. Based on the observed patterns of geochronology, geochemistry and field relationships we suggest that A-type magma genesis in the Eastern Sierras Pampeanas was linked to an Andean-type margin where the lithospheric mantle played a role in its generation.